The aad-12 gene (originally from Delftia acidovorans) encodes the aryloxyalkanoate dioxygenase (AAD-12) protein. The trait confers tolerance to 2,4-dichlorophenoxyacetic acid, for example, and to pyridyloxyacetate herbicides. The aad-12 gene, itself, for herbicide tolerance in plants was first disclosed in WO 2007/053482.
The expression of heterologous or foreign genes in plants is influenced by where the foreign gene is inserted in the chromosome. This could be due to chromatin structure (e.g., heterochromatin) or the proximity of transcriptional regulation elements (e.g., enhancers) close to the integration site (Weising et al., Ann. Rev. Genet 22:421-477, 1988), for example. The same gene in the same type of transgenic plant (or other organism) can exhibit a wide variation in expression level amongst different events. There may also be differences in spatial or temporal patterns of expression. For example, differences in the relative expression of a transgene in various plant tissues may not correspond to the patterns expected from transcriptional regulatory elements present in the introduced gene construct.
Thus, large numbers of events are often created and screened in order to identify an event that expresses an introduced gene of interest to a satisfactory level for a given purpose. For commercial purposes, it is common to produce hundreds to thousands of different events and to screen those events for a single event that has desired transgene expression levels and patterns. An event that has desired levels and/or patterns of transgene expression is useful for introgressing the transgene into other genetic backgrounds by sexual outcrossing using conventional breeding methods. Progeny of such crosses maintain the transgene expression characteristics of the original transformant. This strategy is used to ensure reliable gene expression in a number of varieties that are well adapted to local growing conditions.
Various prior methods can be used to detect the presence of an event in a sample of plant tissue. One example is the Pyrosequencing technique as described by Winge (Innov. Pharma. Tech. 00:18-24, 2000). In this method an oligonucleotide is designed that overlaps the adjacent genomic DNA and insert DNA junction. The oligonucleotide is hybridized to single-stranded PCR product from the region of interest (one primer in the inserted sequence and one in the flanking genomic sequence) and incubated in the presence of a DNA polymerase, ATP, sulfurylase, luciferase, apyrase, adenosine 5′ phosphosulfate and luciferin. DNTPs are added individually and the incorporation results in a light signal that is measured. A light signal indicates the presence of the transgene insert/flanking sequence due to successful amplification, hybridization, and single or multi-base extension. (This technique is usually used for initial sequencing, not for detection of a specific gene when it is known.)
Fluorescence Polarization is another method that can be used to detect an amplicon. Following this method, an oligonucleotide is designed to overlap the genomic flanking and inserted DNA junction. The oligonucleotide is hybridized to single-stranded PCR product from the region of interest (one primer in the inserted DNA and one in the flanking genomic DNA sequence) and incubated in the presence of a DNA polymerase and a fluorescent-labeled ddNTP. Single base extension results in incorporation of the ddNTP. Incorporation can be measured as a change in polarization using a fluorometer. A change in polarization indicates the presence of the transgene insert/flanking sequence due to successful amplification, hybridization, and single base extension.
Molecular Beacons have been described for use in sequence detection. Briefly, a FRET oligonucleotide probe is designed that overlaps the flanking genomic and insert DNA junction. The unique structure of the FRET probe results in it containing secondary structure that keeps the fluorescent and quenching moieties in close proximity. The FRET probe and PCR primers (one primer in the insert DNA sequence and one in the flanking genomic sequence) are cycled in the presence of a thermostable polymerase and dNTPs. Following successful PCR amplification, hybridization of the FRET probe to the target sequence results in the removal of the probe secondary structure and spatial separation of the fluorescent and quenching moieties. A fluorescent signal indicates the presence of the flanking genomic/transgene insert sequence due to successful amplification and hybridization.
Hydrolysis probe assay, otherwise known as TAQMAN (Life Technologies, Foster City, Calif.), is a method of detecting and quantifying the presence of a DNA sequence. Briefly, a FRET oligonucleotide probe is designed with one oligo within the transgene and one in the flanking genomic sequence for event-specific detection. The FRET probe and PCR primers (one primer in the insert DNA sequence and one in the flanking genomic sequence) are cycled in the presence of a thermostable polymerase and dNTPs. Hybridization of the FRET probe results in cleavage and release of the fluorescent moiety away from the quenching moiety on the FRET probe. A fluorescent signal indicates the presence of the flanking/transgene insert sequence due to successful amplification and hybridization.
Another challenge, among many, is finding a suitable reference gene for a given test. For example, as stated in the abstract of Czechowski et al., “An exceptionally large set of data from Affymetrix ATH1 whole-genome GeneChip studies provided the means to identify a new generation of reference genes with very stable expression levels in the model plant species Arabidopsis (Arabidopsis thaliana). Hundreds of Arabidopsis genes were found that outperform traditional reference genes in terms of expression stability throughout development and under a range of environmental conditions.” (Czechowski et al. (2005) Genome-wide identification and testing of superior reference genes for transcript normalization in Arabidopsis. Plant Physiol. 139, 5-17.)
Brodmann et al. (2002) relates to real-time quantitative PCR detection of transgenic maize content in food for four different maize varieties approved in the European Union. Brodmann, P. D., P. D., Ilg E. C., Berthoud H., and Herrmann, A. Real-Time Quantitative Polymerase Chain Reaction Methods for Four Genetically Modified Maize Varieties and Maize DNA Content in Food. J. of AOAC international 2002 85 (3)
Hernandez et al. (2004) mentions four possible genes for use with real-time PCR. Hernandez, M., Duplan, M.-N., Berthier, G., Vaitilingom, M., Hauser, W., Freyer, R., Pla, M., and Bertheau, Y. Development and comparison of four real-time polymerase chain reaction systems for specific detection and quantification of Zea mays L. J. Agric. Food Chem. 2004, 52, 4632-4637.
Costa et al. (2007) looked at these four genes (also in the real-time PCR context) and concluded that the alcohol dehydrogenase and zein genes were the best reference genes for detecting a sample “event” (a lectin gene) for transgenic feed intermix issues. Costa, L. D., and Martinelli L. Development of a Real-Time PCR Method Based on Duplo Target Plasmids for Determining an Unexpected Genetically Modified Soybean Intermix with Feed Components. J. Agric. Food Chem. 2007, 55, 1264-1273.
Huang et al. (2004) used plasmid pMulM2 as reference molecules for detection of MON810 and NK603 transgenes in maize. Huang and Pan, “Detection of Genetically Modified Maize MON810 and NK603 by Multiplex and Real-Time Polymerase Chain Reaction Methods,” J. Agric. Food Chem., 2004, 52 (11), pp 3264-3268.
Gasparic et al. (2008) suggest LNA technology, from a comparison to cycling probe technology, TaqMan, and various real-time PCR chemistries, for quantitatively analyzing maize events (such as MON810). Ga{hacek over (s)}pari{hacek over (c)}, Cankar, {hacek over (Z)}el, and Gruden, “Comparison of different real-time PCR chemistries and their suitability for detection and quantification of genetically modified organisms,” BMC Biotechnol. 2008; 8: 26.
US 20070148646 relates to a primer extension method for quantification that requires controlled dispensation of individual nucleotides that can be detected and quantified by the amount of nucleotides incorporated. This is different from the TaqMan PCR method using an internal reference gene.
To distinguish between homozygous and hemizygous genotypes of TC1507, an Invader assay has been successfully used for this event. Gupta, M., Nirunsuksiri, W., Schulenberg, G., Hartl, T., Novak, S., Bryan, J., Vanopdorp, N., Bing, J. and Thompson, S. A non-PCR-based Invader Assay Quantitatively Detects Single-Copy Genes in Complex Plant Genomes. Mol. Breeding 2008, 21, 173-181.
Huabang (2009) relates to PCR-based zygosity testing of transgenic maize. However, no reference gene appears to be used. Huabang, “An Accurate and Rapid PCR-Based Zygosity Testing Method for Genetically Modified Maize,” Molecular Plant Breeding, 2009, Vol. 7, No. 3, 619-623.